1-Imino nitroxide pyrene 1 has been characterized as a H+ fluorescent switch. The effects of various interfering species, solvents and the irradiation of ultraviolet light on the fluorescence intensity of 1 are also investigated. 1 fluoresces weakly in the 85% ethanol media containing no more than 4.5×10-5 mol/L of H+ (HCl), but can produce a burst increase of about 4times in the fluorescence intensity when the concentration of H+ is increased to or over 4.9×10-5mol/L. The extremely sharp response of 1 to H+ occurs only within such a narrow concentration range. Moreover, the decrease of H+ concentration through adding sodium hydroxide causes the fluorescence quenching, suggesting that 1 may serve as a fluorescent switch for monitoring the concentration change of H+ around the point of 4.5× 10-5 mol/L.